When printed circuit boards ("PCBs") are fabricated, a film or layer of tin and/or tin-lead alloy is typically plated on the conductive copper surface of the board to serve as an etch resist in the subsequent etching away of other copper surfaces. Later in the fabrication process, the tin and tin-lead must be chemically stripped from the copper circuit. One method of stripping tin and tin-lead from copper surfaces uses a single solution containing nitric acid and ferric nitrate. It is known that as tin or tin/lead contacts copper, tin molecules migrate into the copper, forming a copper-tin inner-metallic layer. A stripping solution containing nitric acid and ferric nitrate will remove both the tin or tin/lead, and the copper-tin inner-metallic layer. The nitric acid strips the pure tin or tin-lead off the panel, and ferric nitrate is used to aid in stripping the remaining copper-tin inner-metallic layer to bare copper.
However, problems arise when using nitric acid/ferric nitrate tin and tin-lead stripping solutions. As described in more detail below, these problems include negative effects on the appearance of the underlying copper, excessive sludge formation, potential for an exothermic condition, excessive attack on the underlying copper, and the evolution of toxic NOx gas from the nitric acid while in the presence of the tin and copper ions. To correct these problems, different materials have been added to the nitric acid/ferric nitrate stripping solution to stabilize the solution. However, the added material often increase the cost of the stripping process. In some cases, the added materials are so expensive the resultant stripping solutions are impractical to use.
As described above, when the tin or tin/lead etch resist is stripped, the copper circuit is exposed on the dielectric material. For aesthetic and functional reasons, it is important that the copper surface resulting from the stripping operation have a uniform, reflective, bright pink appearance. For example, the circuit board can then be inspected using automated optical inspection, or AOI. AOI uses reflected light from the dielectric material and copper circuit to inspect the boards. Because the dielectric material is dull and non reflective, if the copper surface is uniform, reflective, and bright pink, AOI can easily distinguish between the dull, non-reflective dielectric material and the uniform, reflective, bright pink copper. Therefore, a uniform, reflective, bright pink copper surface increases the effectiveness of the AOI. Having a copper surface which is a non-uniform matte pink finish decreases the effectiveness of AOI.
Sludge formation is another problem associated with conventional tin and tin/lead nitric acid-based stripping. During the stripping process, tin and tin/lead stripping solutions contain dissolved metals. As the metal loading of the stripping solution increases, the metal precipitates out of solution as sludge. It is believed that the sludge precipitate is stannic oxide. In spray applications, the sludge can cause clogging in the spray nozzles. When the stripping solution is sprayed onto the circuit boards to strip the tin or tin/lead, sludge is also sprayed onto the boards. This sludge may be difficult to rinse off of the circuit board, leaving a white, chalky material on the board. Sludge also makes it difficult to keep conveyerized process equipment clean.
During a nitric acid-based tin and tin/lead stripping process, when a certain metal loading level is reached, the stripping solution often becomes unstable, creating a potential for an exothermic condition, which is an instant release of a massive amount of heat. Typically, large amounts of toxic NOx gas are released during an exothermic condition, and the striping solution foams excessibely. Furthermore, the temperature of the stripping solution can increase to 150.degree. F. or greater. Obviously, the occurrence of exothermic conditions can damage the operating equipment.
Another problem caused by conventional nitric acid-based tin and tin/lead stripping solutions is an excessive copper attack rate. Obviously it is desirable to minimize copper attack rate during tin or tin/lead stripping. A high copper attack results in stripping the copper off the circuit and exhausting the strength of the tin and tin/lead stripping solution on unnecessary copper removal. Also, process consistency is not achieved when a stripping solution has an initial high copper attack rate which drastically decreases with metal loading.
Evolution of toxic NOx gas during the stripping process poses a critical safety problem associated with nitric acid/ferric nitrate-based stripping solutions. Toxic NOx gas evolves from the nitric acid while in the presence of the tin and copper ions. The NOx evolving from the reaction between nitric acid and tin or tin/lead is any mixture of nitrogen dioxide and nitrogen monoxide. Both nitrogen dioxide and nitrogen monoxide are considered poisonous gases by the Occupational Safety and Health Administration. OSHA has set exposure limits on these toxic gases; the present permissible exposure level of nitrogen dioxide is 3 ppm, and the time weighted average of exposure for nitrogen monoxide is 25 ppm.
Various methods and compositions have been developed in attempts to stablize nitric acid-based stripping solutions so as to prevent the above-described problems. For example, U.S. Pat. Nos. 4,957,653 and 5,017,267 to Cordani disclose the addition of an alkane sulfonic acid such as methane sulfonic acid, to form highly water-soluble salts of the dissolved metals. However, the expense of the additional raw material leads to an expensive tin and tin/lead stripping process. A more recent Cordani patent (U.S. Pat. No. 5,234,542) uses sulfuric acid to stabilize the nitric acid stripping solution, thus inhibiting copper attack. However, the other problems, such as high levels of toxic NOx emmissions, remain.
U.S. Pat. No. 4,713,144 to Schiller discloses the use of sulfamic acid to stabilize the nitric acid stripping solution and to inhibit copper attack. However, the Schiller solution causes excessive sludge formation, and has a high potential for creating an exothermic condition. Furthermore, the Schiller method and composition leaves a non-uniform, matte pink finish on the underlying copper surface, rather than the desired uniform, reflective, bright pink appearance.
U.S. Pat. No. 4,374,744 to Kawanabe principally addresses the problem of copper attack. The Kawanabe patent discloses stripping solutions consisting of an inorganic and/or organic acid, an oxidizing agent, and a heterocyclic compound free of sulfur but containing a nitrogen atom in the form of .dbd.NH or .tbd.N as a ring forming member. Examples given include imidazole and derivatives thereof and triazoles and derivatives thereof. The Kawanabe patent lists general categories of compounds expected to be suitable in inhibiting copper attack, such as pyrroles, pyrazoles, imidazoles, and triazoles. However, not all compounds in the Kawanabe category of "heterocyclic compound free of sulfur but containing a nitrogen atom in the form of .dbd.NH or .tbd.N as a ring forming member" function to inhibit copper attack, and different compounds within the same chemical category exhibit different behavior regarding inhibiting copper attack. Not only do many of the Kawanable solutions cause excessive sludge formation, but, more critically, most such solutions release approximately 100 to 1000 ppm of NOx gas during stripping, orders of magnitude higher than the level of NOx deemed acceptable by OSHA, as evidenced by the visual orange NOx fumes observed during the stripping process.